In accordance with spread of a portable audio player, a noise reduction system has begun to spread, which is designed for a headphone or an earphone for the portable audio player and which reduces noise of an external environment to provide favorable reproduction sound field space in which external noise is reduced, to a listener.
For example, Patent Literature 1 discloses a noise reduction apparatus which converts an analog signal of external noise obtained by being collected at a microphone into a digital signal, generates a noise reduced signal for reducing the external noise using the digital signal and applies the noise reduced signal to an audio signal.